Soldering is a general technique for mounting electronic components on the surface of a printed circuit board. However, soldering produces corrosive flux residues that decrease the quality of the printed circuit substrate. For this reason, soldering must be followed by a process for cleaning the substrate, so as to remove the residues.
Cleaning of various electric components or alloy components have heretofore been performed using chlorinated cleaners containing chlorinated solvents, such as trichloroethylene, perchloroethylene, methylene chloride or the like as the main components. Such chlorinated solvents are advantageous in that they are nonflammable and have excellent drying properties. However, the use of chlorinated solvents is currently restricted because they cause ozone layer depletion, soil pollution and like environmental problems; and they are toxic to humans; etc. Further, in recent years, ensuring the cleaning effect against ionic residues such as chlorine ion, sulphate ion etc. has been particularly required in the cleaning of electric components etc.; however, chlorinated solvents have little ability to clean ionic residues.
Furthermore, in recent years, people have become aware of the hazards of lead, and the use of solders containing lead has been restricted in many countries. Accordingly, lead-free solder has been increasingly used. When a lead-free solder is used, the soldering temperature has to be set at least 30° C. higher than the case where a known tin-lead eutectic solder is used, except for some cases where specific alloy-based lead-free solders are used; additionally, the solder wettability of lead-free solder is inferior to that of known eutectic solder. In order to solve such problems, a stronger activator than that normally used in lead-free soldering flux is used. Accordingly, the flux used for lead-free solder is more likely to produce reaction products during the soldering process than the known tin-lead eutectic solder.
The reaction product is a tin salt compound, such as a salt of activator, contained in the flux used for lead-free solders, and tin or a salt of resin acid and tin. These tin salt compounds are bivalent or tetravalent tin salts, which have significantly poor solubility to cleaners. Therefore, when cleaning of lead-free solder is conducted using a known halogen-free cleaner composition or a polar or non-polar organic solvent, due to its insufficient ability to remove tin salt compounds, flux residues derived from the tin salt remain on the surface of the cleaned object, or pollutants derived from the tin salt will re-attach to the surface of the cleaned object.
When the object to be cleaned is a sheet-type article, such as a mounted board, it is common to use a spray to apply the cleaner and the rinsing liquid (water, in general) to the object so as to improve efficiency during the cleaning process and water-rinsing process. For example, some Patent Literatures disclosed a method using a cleaner containing a surfactant that is used in the form of a spray (Patent Literatures 1 and 2). However, when water is sprayed to the object after the object is cleaned using the cleaner of Patent Literature 1 or 2, a large amount of foam is generated on the surface of the object. This causes some complications in the management of the water-rinsing process. An antifoaming agent may solve this problem; however, the use of an antifoaming agent may result in improper washing.
In addition, there has been a demand for a cleaning system that can reduce the amount of waste fluid in terms of reduction of environmental burden. However, in the reservoir rinsing method used for the rinsing process in the semi-aqueous flux cleaning system, washing solution and flux resulted from the cleaning step are accumulated in the pre-rinsing water in the pre-rinsing tank, and when the concentration reaches a certain level that may influence the surface quality of the object to be cleaned and the water quality of the post-rinsing water, all of the rinsing water is discarded. The amount of pre-rinsing waste fluid thus generated is actually larger than the consumption amount and disposal amount of the cleaner used in the general cleaning and water-rinsing step using a spray. Hence, this large amount of waste fluid has posed a great environmental burden. Further, since all of the pre-rinsing water is also discarded as pre-rinsing waste fluid, it is necessary to replace all of the pre-rinsing water with fresh water. Therefore, if the cleaning process is performed with a cleaner composition that can be easily separated from the pre-rinsing water, it becomes possible to completely remove only the cleaner composition. This allows reuse of pre-rinsing water, thereby reducing the frequency of replacement of pre-rinsing water with fresh water. This should be conducive to reduction of environmental burden.